Receptors located in the heart and coronary arteries with afferent nerve fibers within sympathetic nerve tracts and vagus nerve tracts have been demonstrated histologically and physiologically. Some afferent nerves conducting neural information from these receptors change their discharge frequency and patterns of discharge coincident with alterations in volume or pressure within the heart chambers or within major thoracic veins, aorta, and pulmonary artery. Other receptors change their discharge patterns during myocardial ischemia. The primary purpose of this research proposal is threefold: 1) To establish the neuroanatomic basis for certain cardiac reflexes by studying the normal ultrastructure and fiber composition of cardiac nerves known to contain vagal and sympathetic afferent and efferent nerves. The afferent fibers in these nerves will be identified selectively through Wallerian degeneration produced by specific nerve sectons such as dorsal root ganglioectomy. 2) To determine the mechanisms for reflex activation including adequate stimulus and stimulus response characteristics of intracardiac and coronary artery mechanoreceptors and ischemia sensitive receptors. 3) To determine changes in sympathetic and vagal efferent nerve activity and reflex responses to activation of cardiac receptors. Afferent nerve activity will be recorded in dissected single fiber and small multifiber preparations of the cardiac vagus and cardiac sympathetic nerves. The time course of receptor activation will be studied by simultaneous recording of afferent nerve activity and intracardiac and aortic pressures, aortic flow and ECG in the dog and monkey. Changes in vagal and sympathetic efferent nerve activity will be studied in nerves conducting impulses to the heart as well as selected peripheral autonomic nerves. Cardiac effects produced by activation of the reflexes will be studied by measuring ventricular function as well as studies of cardiac rate and rhythm. Changes in cardiac conduction will be evaluated using multiple cardiac electrodes. Peripheral circulatory effects of cardiac reflexes will be evaluated by studying pressure flow change in selected peripheral circulatory beds including the kidney, lung, hind limb and splanchnic circulation.